52 Life and TIimmortality. 
posterior end of the body. But this curious structure, as 
shown by Claparéde, merely consists of a deep longitudinal 
involution of the walls of the intestine, by which means an 
extensive absorbent surface is secured. 
Worms have a well-developed circulating system. Their 
breathing is effected by the skin, and so they do not possess 
any special respiratory apparatus. Each individual unites 
the two sexes in its own body, but two individuals pair 
together. The nervous system is fairly well developed, the 
two nearly confluent cerebral ganglia being situated very 
close to the anterior extremity of the body. 
Being destitute of eyes, we would naturally conclude that 
worms were quite insensible to hght; but from many experi- 
ments that have been made by Darwin, Hofmeister and 
others, it is evident that light affects them, but only by its 
intensity and duration. It is the anterior extremity of the 
body, where the cerebral ganglia lie, that is affected, for if 
this part is shaded and other parts of the body are illumi- 
nated no effect will be produced. As these animals have no 
eyes, it is probable that the light passes through their skins 
and excites in some manner their cerebral ganglia. When 
worms are employed in dragging leaves into their burrows 
or in eating them, and even during the brief intervals of rest 
from their labors, they either do not perceive the light or are 
regardless of it, and this is even the case when the light is 
concentrated upon them through a large lens. Paired indi- 
viduals will remain for an hour or two together out of their 
burrows, fully exposed to the morning light, but it appears, 
from what some writers have said, that a light will occasion- 
ally cause paired individuals to separate. When a worm is 
suddenly illuminated and dashes into its burrow, one is led 
to look at the action as a reflex one, the irritation of the 
cerebral ganglia apparently causing certain muscles to con- 
tract in an inevitable manner, without the exercise of the 
will or consciousness of the animal, as though it was an 
automaton. But the different effect which a light produces 
